This invention relates to citrus waste processing and, more particularly, a method for the conversion of simple and complex carbohydrates contained in solid citrus waste into ethanol for use as bio-fuel and to yield other high-value byproducts.
Currently, the generation of solid citrus waste, consisting primarily of peel, membranes, and seeds, which result from the processing of citrus fruit for juice, is an environmental problem. The problem exists particularly in areas where the bulk of citrus is grown to produce juice, such as in the State of Florida and the country of Brazil. For example, Florida in 2003 had approximately 103 million citrus trees on 800,000 acres and produced 297 million boxes of citrus, 85% of which was processed into juice. The waste from such processing was approximately one-half of the citrus fruit, yielding approximately 5 million tons of wet solid waste which reduces to 1.2 million tons of dry waste. While raw wet solid citrus waste contains between 76-82% water, the spongy structure of the citrus waste and the high content of polysaccharides in the form of pectin, cellulose and hemicellulose binds this water so the solid citrus waste material consists of solid pieces with no free liquid between the solid pieces. The composition of citrus waste is approximately 5-8% simple sugars (e.g., fructose, glucose, sucrose), 3-5% pectin, 2-3% cellulose, 2-3% hemicellulose, 1-2% flavonoids, 1% organic acids, 1% protein, 1% ash, and 1% oil. Traditionally, such solid citrus waste has been converted into cattle feed which typically does not have sufficient value to cover the production and transportation costs associated therewith. A further drawback of converting current citrus waste into cattle feed is that the waste contains a high amount of d(+)-limonene (generally referred to simply as limonene). Volatilization of the limonene during the drying process causes air pollution to the extent that limonene vapors are exhausted into the atmosphere at the processing plants, this happens because it would require very expensive equipment to trap the limonene from the drier exhaust. Although citrus waste materials do create an environmental problem, these materials are rich in pectin and other polysaccharides that can be hydrolyzed into sugars for use in the production of ethanol.
Currently ethanol is used as a bio-fuel that is mixed with gasoline to increase the octane rating and improve the environmental characteristics of gasoline. Although another gasoline octane enhancer referred to as MTBE (Methyl Tertiary Butyl Ether) is also used, utilization of MTBE is controversial since it is not biodegradable, results in ground water pollution and for these reasons its use has already been banned in more than 17 states with more states expected to follow. Field corn (maize) is currently the primary feedstock for ethanol production in the USA. As the State of Florida has no cultivation of field corn, Florida must look to other sources for producing ethanol. The conversion of solid citrus processing waste into ethanol would reduce waste and provide a regional source of ethanol as a viable alternative octane enhancer to MTBE. The conversion of citrus processing waste in 2003 of approximately 5 million tons could result in potentially 100 million gallons of ethanol.
A major problem that prevents processing citrus waste into ethanol using microorganisms is limonene. Limonene is a liquid terpene that is contained in citrus peel. Limonene provides a natural defense for the fruit against bacteria, viruses, molds, and other organisms. Accordingly, limonene helps to protect the citrus waste from microbial buildup and fermentation by normal processes that would yield ethanol. It is also desirable to recover the limonene as a high value co-product. For efficient fermentation, limonene in the citrus waste must be reduced to a level below 3000 parts per million (preferred level below 1500 ppm) from the natural level of about 5,000-15,000 ppm. Another problem with fermentation of raw or pressed citrus waste is contamination with bacteria and other microorganisms which can take over the fermentation and divert it to products other than ethanol. Thus, a need exists for processes that will decrease the amount of limonene in citrus processing waste and sterilize or pasteurize the waste in order to produce ethanol for use as a bio-fuel and also to produce other high value products, including cattle feed, limonene, five carbon sugars and galacturonic acid monomers and polymers.
Yet another problem is the release of fermentable sugars from citrus waste at concentrations as high as possible. Ethanol is produced at approximately 50 weight % yield based on sugars consumed, and the energy consumption for recovery of ethanol by distillation rapidly increases with decreasing ethanol concentration in fermented mixtures (mash). Dilution of solid citrus waste with large amounts of water often currently used to aid processing and to accommodate equipment limitations is thus not practical for ethanol production from this material. Citrus waste needs to be processed at high concentration of solids which makes handling and processing of the material very difficult; in addition, waste tissues need to be broken down to release soluble sugars and complex carbohydrates (e.g., pectin, cellulose, hemicellulose) must be hydrolyzed to release additional sugars and maximize ethanol production.
Some individual steps such as shredding, limonene removal, hydrolysis of the citrus waste, and fermentation of sugars to ethanol are known in the prior art. However, the prior art has not provided an integrated system or method which can process solid citrus waste with minimal dilution by water and which addresses all of the problems outlined above.
U.S. Pat. No. 4,503,079 (King) discloses a process for the production of ethanol from citrus press liquor and molasses. Citrus press liquor is a liquid product obtained by treating solid citrus waste with lime and pressing it to release the juices (citrus press liquor). The citrus press liquor is usually concentrated by evaporation to another liquid product called citrus molasses. The limonene contained in press liquor can be mostly removed during evaporation in overhead vapors as disclosed in U.S. Pat. No. 2,561,072 (Reich). King discloses removal of limonene (peel oil) from citrus press liquor by steam stripping at high temperatures (240°-260° F.), dilution of deoiled citrus molasses with water and fermentation of such deoiled, diluted molasses to ethanol. The process disclosed by King does not teach removal of limonene from solid citrus waste and is unsuitable for processing of such material since the equipment utilized by King is incapable of handling solid citrus waste. In addition the process disclosed by King is very inefficient in that it utilizes less than one half of the soluble fermentable sugars present in citrus waste as only those fermentable sugars present in the liquid portion that can be pressed from the waste are used and the press liquor yield is approximately one half of the total liquid contained in the citrus waste. None of the fermentable sugars in the complex carbohydrates are utilized by King because these carbohydrates are not hydrolyzed and released and made available for fermentation. The method described by King requires liquid streams and cannot handle solids or highly viscous materials whereas the present invention described herein can handle solid citrus waste and the entire citrus waste stream taking advantage of all the sugars present, including those in the form of complex carbohydrates.
Pavilon in U.S. Pat. Nos. 4,952,504 and 5,135,861 discloses a chemical method for hydrolysis of citrus waste at very high temperatures (400°-500° F.) and pressures (365-515 psia) using carbon dioxide (carbonic acid) or indigenous acids as catalysts. However, the process disclosed by Pavilon can only utilize and process the whole citrus waste biomass as a slurry in water; about one-fourth to one-third of a gallon of water (approximately 2-3 pounds) is added for each pound of peel. This method thus relies on addition of water and excessive dilution (3-4 fold) of the initial solid citrus waste material and results in a fermented material (mash) with ethanol contents of less than 1.5%. However, distillation costs to recover ethanol from such fermented mash with less than 4% ethanol by volume rise exponentially and quickly becomes prohibitively expensive (The Alcohol Textbook, 2003, K. A. Jacques, T. P. Lyons, D. R. Kelsall, Ed., Notingham University Press, Notingham, UK, p 326.). The method described by Pavilon requires slurries and cannot handle solids or highly viscous materials. Pavilon also discloses that the citric acid content present in citrus peel can be used to catalyze the hydrolysis of the pectin, cellulose and hemicellulose into simple sugars at the temperatures used. However, citric acid is not present in the waste as a free acid but as salts and the concentration of catalytic hydronium ions is too low (pH=4-5) for efficient hydrolysis of these complex polysaccharides; thus only incomplete hydrolysis is possible. Pavilon does not disclose removal of limonene prior to fermentation and instead relies on dilution of citrus waste with water to decrease limonene concentration in the fermentation step.
U.S. Pat. No. 6,251,643 (Hansen) discloses a process for hydrolysis of biomass (e.g., vegetable) using a multistage screw press to carry out the process. While the screw press reactor may be suitable for processing of fibrous vegetable matter, it is not suitable for processing of steam (vapor) treated citrus processing waste since such waste becomes extremely soft upon steaming and passes easily through screen openings even under very low pressure. Screen presses or similar filtering devices are thus unsuitable for dewatering of heat treated, wet highly viscous citrus waste. Hansen also relies on addition of large amounts of liquid (aqueous solutions) and subsequent pressing to accomplish heating, cooling and separation of vegetable matter prior to enzymatic hydrolysis. Such addition of water dilutes the concentration of sugars and ultimately ethanol. In addition, screw press devices are complex, consume large amounts of energy for pressing, and have high maintenance requirements. The present invention does not require separation of liquids and solids until the last steps, i.e. after fermentation or distillation have been completed; the recycling of large streams of liquid is thus avoided, equipment is simplified, and a more efficient method is provided.
The present invention disclosed herein utilizes all fermentable sugars, both free and those bound as complex carbohydrates (e.g., cellulose, hemicellulose, and pectin) using physical and chemical hydrolysis aided by enzymes and does not require addition of excessive amounts of water. The present invention provides a means of economically treating and handling solid citrus waste with minimal dilution so that the solids content of the original solid citrus waste substrate is reduced by less than about 25% (e.g., less than 25%; preferably less than about 10% (e.g., less than 10%), more preferably less than about 5% (less than 5%)), compared to the raw feed, to yield a substrate and means to handle the substrate with means of liquefying and fermenting such substrate that after fermentation yields a product containing about 4% to about 6% (e.g., 4-6%) ethanol from which the ethanol can be economically stripped to produce a beverage or fuel grade product. The added water comes only from the difference between the steam utilized and condensed in the jet cooker and the water evaporated during flashing of limonene and vacuum cooling. This typically amounts to addition of less than about 0.3 lbs (e.g., less than 0.3 lbs; 0.036 gallons) of water to each pound of added citrus waste, preferably addition of less than about 0.15 lbs (e.g., less than 0.15 lbs; 0.018 gallons), and more preferably less than about 0.1 lb water (e.g., less than 0.1 lbs; 0.01 gallons) for each pound of added solid citrus waste. This is at least 7 to 30 times less water addition than is necessary in the process described by Pavilon in U.S. Pat. Nos. 4,952,504 and 5,135,861.
The present invention disclosed herein, for processing solid citrus waste to ethanol, utilizes enzyme mixtures (e.g., pectinase, hemicellulases, cellulases and beta-glucosidases) for efficient hydrolysis of the complex carbohydrates in citrus waste residue into simple sugars with concurrent fermentation or hydrolysis followed by fermentation. It offers several advantages over the prior art. The process described by King does not utilize all of the sugars available (less than ⅓) in citrus waste as only the part of the liquid portion that can be pressed from the waste to produce press liquor and concentrated citrus molasses is used. The process described by Pavilon claims to utilize all the sugars in the waste stream but requires addition of excessive amounts of water to allow handling of the citrus waste and decrease inhibition by limonene, resulting in a fermented product with a dilute ethanol content under 1.5% which makes it substantially more costly to recover the ethanol.
Two different processes, (1) gas (e.g., flue gas, air) or steam stripping (preferred) or (2) centrifuging, can be used in the present invention to lower the limonene content in the citrus waste to a sufficiently low level whereby subsequent fermentation of the waste can efficiently produce ethanol. Steam stripping is most preferred as the waste does not have to be hydrolyzed or diluted with water as required for centrifuging, and the removed limonene may be easily collected and recovered by condensing the stripped water and limonene vapors with the limonene collecting on top of the water layer. The fermentation utilizes traditional ethanol producing yeast, E. coli strain KO11, or other bacteria or fungi, followed by distillation to recover ethanol. The solids residue remaining may still be utilized as a cattle feed product, having a higher protein content than the citrus-based cattle feed currently being produced, and retaining the health promoting flavonoids contained in citrus peel. Alternatively, the solids residue after distillation may also be pressed and filtered with optional recovery of acetate, five carbon sugars, or galacturonic acid monomers/polymers from the filtrate. Both steam stripping or centrifugation and hydrolysis work more efficiently if the raw solid citrus processing waste is ground to a particle size of less than about one inch (e.g., less than one inch; preferably less than about one-half inch (e.g., less than one-half inch)) using a hammer mill, grinding pump or similar shredding/chopping/grinding apparatus capable of handling and reducing said waste to the required size; such equipment is well known in the art. A progressing cavity pump or similar pump (or conveyor) capable of pumping/moving raw or ground peel waste with dry solids content up to thirty-five percent is then used to feed and mix the material (resulting from steam stripping and cooling) during the enzymatic hydrolysis and fermentation.
In a preferred processing embodiment the particle size of solids in the raw solid citrus waste is reduced to a size sufficient for further processing, then the ground peel is heated to a range of about 80° to about 240° C. (e.g., 80° to 240° C.; preferred range about 100° to about 160° C. (100° to 160° C.)) by direct steam injection (et cooker), passage through a heated hollow shaft screw conveyer or other direct or indirect heating device at a pressure between about 0 to about 150 psi (e.g., 0-150 psi; preferred range about 10 to about 100 psi (e.g., 10-100 psi), more preferred range about 20 to about 80 psi (e.g., 20-80 psi)), and then released into a flash tank at a reduced pressure where the limonene and condensed steam vaporize and are removed to the condenser. Heating by steam injection and sudden decompression has the benefit of a simultaneous or sequential shearing and disintegration action which is beneficial to the subsequent enzymatic hydrolysis process. The heating causes the limonene content to be decreased through evaporation and steam stripping with additional limonene being removed during vacuum cooling. The limonene is then recovered by condensation of the removed steam and decanting (or centrifuging) the recovered liquid to separate immiscible water and limonene layers. The solid or semi-solid and highly viscous citrus waste is then cooled and adjusted for pH, followed by sequential or simultaneous hydrolysis and fermentation using an enzyme mixture and fermentation organisms such as yeast, E. coli strain KO11, or other bacteria or fungi, in an agitated system with mixing accomplished using high solids pumps or high solids mixers to facilitate hydrolysis and fermentation. After fermentation, the ethanol is separated by distillation and the resulting residue can then be pressed and dried for use as cattle feed or further processed with fermentation using E. Coli KO11, or other bacteria or fungi, to produce more ethanol and acetate, or the unfermented galacturonic acid monomers/polymers and five carbon sugars may be recovered as additional products. While the method described herein is unique in converting solid citrus juice processing waste which is high in pectin and other complex carbohydrates, making it difficult to handle, it can also be used to convert other processing waste materials with chemical compositions similar to citrus waste, such as sugar beet bagasse, containing large amounts of pectin and which are also difficult to handle in processing without adding large volumes of water.